These are all questions you should address, since the supply of models is limited, unlike a scale like HO, where virtually everything is available.

Basically, you want to decide if scale is important to you, and how important.

(If you are coming from another scale, the following stuff will seem strange, but read on)

#1 Gauge, 45 mm between the tracks, would be represented by a scale of 1:32 for standard gauge, and 1:20.3 for most American narrow gauge.

But, scales available that run on #1 gauge track are 1:20.3, 1:22, 1:24, 1:29 and 1:32 (mostly) Within these scales, many different types of products are available.

If you are into narrow gauge steam locomotives, you are in luck, since there are a number of good models available, all narrow gauge. Many of them are somewhat expensive, but there are some good inexpensive ones available from Bachmann. The higher priced stuff is from Accucraft, and a few others. The "reasonably priced" stuff is from Bachmann. The 45 mm track works out to the right size for narrow gauge in 1:20.3, pretty much the standard narrow gauge you can get in the US.

If you are into standard gauge trains, and exact scale is important, you have the possibility to buy equipment, but your costs are going way up. MTH makes/made a nice line of models in 1:32 at OK prices, but they are about the only "reasonably priced" alternative in 1:32, and also a lot of "liberties" have been taken to allow locomotives navigate "toy like" sharp curves. (This is from the MTH heritage in O scale). MTH is not terribly active in the #1 scale, and no new locomotives in a while. They have very nice sound systems, and a nice (but non-standard) remote control. You cannot buy the locos without the extra electronics. The good news is that recently the Protosound III electronics will work with DCC. Your only other locomotive and rolling stock choices are much more expensie, Aster and several other high end manufacturers make beautiful models, but these can run into thousands of dollars for a locomotive. Basically be prepared to spend more on cars and locos.

If you are into standard gauge, and exact scale is not so important, USA trains, Aristocraft and AML (American Main Line made by Accucraft) make excellent inexpensive models in 1:29. The trucks are a bit narrower than scale, since the track should be theoretically wider. Most people accept this in exchange for the large selection and good prices. More and more companies are build in this scale, like Gibert LeCroix' GLX Models.

The 1:29 stuff came about sort from the "wow" factor. If your layout is on the ground, the 1:29 standard gauge locomotives look much larger than the exact scale 1:32 stuff. There is also more available in this scale than any other.

(of course, if you are in narrow gauge, then the 1:20.3 is even larger, more wow!)

If you do not worry too much about scale, you can mix various scales, but right next to each other they can look funny to most people.

The oddball scales look pretty strange when mixed with other scales.

Worth mentioning here is LGB, who pretty much is responsible for the popularity of this part of the hobby. They often have products in a compromise scale in terms of width and height, but shortened to run on their tightest curves. Next to a 1:29 model, they often look short and toylike but wider and taller. This is excellent quality stuff, but if you want scale appearance, this won't be your favorite. They have gone into bankruptcy twice in recent years.

My best advice is go to someone's layout, have them put the different scales next to each other, look at the trains and see what floats your boat!